Wireless links, such as radio frequency (RF) links, are becoming widely used in providing communication of information, including images, audio, and/or data. For example, wireless local area networks (WLANs) have recently become available as an alternative to wired local area networks. With a WLAN, wires are replaced with wireless links between an access point (AP) and wireless devices. Wireless devices may include desktop computers, portable computers, personal digital assistants (PDAs), printers, servers, and/or other devices that exchange information via wireless links.
An access point typically transmits and receives data to and from a wireless device within a coverage area. The coverage area may correspond to an office, house, building or other area of operation. Each wireless device may exchange information with the access point while in the coverage area and contends for the bandwidth provided by the access point with other wireless nodes (e.g., wireless devices and/or access points) in the coverage area. Accordingly, protocols are typically implemented, such as at the media access control (MAC) layer to resolve communication link contention.
For example, the IEEE 802.11 media access control standard proposes a random access technique, also widely implemented in wired networks, which uses a carrier sense with collision avoidance (CSMA/CA) scheme. Pursuant to CSMA/CA, a wireless device waits until no other wireless device is transmitting on a chosen channel and then transmits data. If a collision occurs, the transmitting device backs off a random interval of time and begins again if the channel is clear. Pursuant to this technique, individual wireless devices eventually get access to the channel that the device is trying to access.
However, a carrier sense with collision avoidance technique alone is not well suited for dealing with the problem of hidden-nodes, e.g., wireless devices which have a sufficiently clear communication path with a common access point but which are positioned so as to have communications between the wireless devices themselves blocked or outside of the other wireless device's radiation pattern (antenna beam). In the hidden-node situation, a first and second node may each perceive the communication channel to an access point as clear, although the other node is transmitting. Simultaneous transmission by both such nodes can result in interference blocking both node's communications, with even the above mentioned waiting a random interval of time and retransmitting the message resulting in repeated overlapping communications and further blockage.
Request-to-send/clear-to-send (RTS/CTS) exchange protocols have been developed for use in addressing the hidden-node problem of the CSMA/CA protocol in the IEEE 802.11 MAC specifications. The use of RTS/CTS protocols, however, does not address the instances in which RTS or CTS packets themselves collide. For example, a collision window remains with respect to uplink transmission of the RTS packet, wherein this collision window is of a time corresponding to the RTS packet length. Moreover, transmission of RTS and CTS packets consumes bandwidth which might otherwise be available for transmission of payload using the wireless communication links. Additionally, the use of RTS/CTS requires adaptation of all the nodes to accommodate the RTS/CTS transmissions and to invoke the protocol.
Another prior attempt to address the hidden-node problem has included the use of a repeater station, as shown in U.S. Pat. No. 6,539,028 to Soh et al., the disclosure of which is incorporated herein by reference. The repeater comprises a demodulator, a modulator and a power level monitor. When the repeater receives wireless communications on an uplink channel, the repeater retransmits the wireless communication on a downlink channel, wherein the downlink channel has a different transmission wavelength than the uplink channel. The wireless nodes perform collision detection by comparing a delayed form of their transmitted bit pattern with the bit pattern received from the repeater. Accordingly, the use of the foregoing repeater requires added hardware in the form of the repeater itself. In addition, the repeater may be utilized only with respect to frequency division duplex communication systems, as separate uplink and downlink wavelengths are required.
There is a need in the art for an efficient technique for avoiding communication transmission collisions in a wireless network, such as a WLAN, that does not require adaptation or alteration of each wireless node thereof. There is a further need in the art for a technique for avoiding communication transmission collisions which does not consume otherwise available bandwidth.